1. Field of the Invention
The present invention relates to flame retardant compositions useful for coating electrical and electronic devices.
2. Brief Description of Related Technology
Epoxy resins are widely used in coating powder compositions for coating electrical and electronic devices. Such epoxy coating powder compositions used for encapsulation are generally prepared from a blend of an epoxy resin and a phenol hardener, along with other ingredients including fillers, catalysts, flame-retardant materials, processing aids and colorants. Epoxy resins in such molding compounds are traditionally diepoxides which include two epoxy groups per molecule, which are reacted with a co-reactant (cross-linking agent or hardener) consisting of acid dianhydride, diamine or diphenol oligomers. Diphenol oligomers, such as those derived from novolac phenols, cresol phenols and bisphenol A, are particularly preferred in the art as hardeners due to their high reliability.
Flame-retardants in epoxy compositions are typically provided for safety purposes. A common flame-retardant system is a combination of bromine-containing flame retardants and antimony oxide flame-retardant synergists. However, these compounds are pollutants of the environment. Some bromine-containing flame retardants (especially brominated diphenyl ethers) are toxic and possibly carcinogenic. Antimony trioxide is classified by the International Agency for Research on Cancer as a Class 2B carcinogen (i.e., antimony trioxide is a suspect carcinogen based mainly on animal studies). In addition, this compound is often used at a relatively high level (2-15%)and is also slightly water-soluble, leading to further environmental concerns. This concern is highlighted by the fact that electrical and electronic manufacturers currently discard up to one half of the total amount of powder coating compositions used.
Phosphorus-containing compounds have been proposed as flame retardants. For example, U.S. Pat. No. 5,739,187 to Asano et al. discloses epoxy resin compositions as semiconductor encapsulants, which include a phosphorus-containing flame retardant to eliminate the use of antimony trioxide and brominated compounds. However, coating powder compositions containing conventional phosphorus compounds generally possess undesirable properties such as high moisture absorption, which can cause stress and cracking of the encapsulant at elevated temperatures.
Melamine cyanurate is commonly sold as a flame-retardant compound. Although effective as a flame retardant, high levels of this material oftentimes severely reduce the flowability of coating powder compositions. As a result, it has generally been considered impractical to incorporate melamine cyanurate into coating powder compositions at appropriate levels to achieve both adequate flame retardancy and flowability. Unfortunately, reducing the amount of the flame retardant to address the flowability issues compromises flame retardance, with the resulting coating powder compositions failing to meet the Underwriters Laboratories Inc. flame retardance standard, UL-94 V-O rating.
U.S. Pat. No. 6,097,100 discloses a semiconductor element encapsulated in a flame resistant resin composition containing an organic compound selected from the group consisting of organobromine compounds, organophosphorus compounds and organonitrogen compounds, an inorganic filler and a metal borate.
It would be desirable to provide flame-retardant coating powder compositions that overcome the aforementioned problems while providing commercially acceptable physical properties. Moreover, it would be desirable to provide coating powder compositions having good flowability and stress characteristics such as good thermal shock resistance while providing acceptable flame retardance under UL-94.